Phenol-formaldehyde novolac resins can be produced by reacting a molar excess of phenol with formaldehyde in the presence of an acid catalyst, such as sulfuric acid, hydrochloric acid or, oxalic acid (usually in an amount of 0.2 to 2% by weight based on the phenol). To prepare the so-called “high ortho” novolac resins, the strong acid catalyst is typically replaced by a divalent metal oxide (e.g., MgO and ZnO) or an organic acid salt of a divalent metal (e.g., zinc acetate or magnesium acetate) catalyst system. In either case, maintaining a molar excess of phenol, by maintaining the mole ratio of phenol to formaldehyde at, for example, 1:0.7-0.9, is common when preparing such resins.
The novolac resins so-produced are thermoplastic, i.e., they are not self-crosslinkable. Such novolac resins are converted to cured resins by, for example, reacting them under heat with a crosslinking agent, such as hexamine (also called hexa or hexamethylenetetramine), or for example, by mixing them with a solid acid catalyst and paraformaldehyde and reacting them under heat. Novolac resins also may be cured with other cross linkers such as resoles and epoxies.
Novolac resins have many uses. One major use of such resins is as a binder in the foundry industry for making sand molds and sand cores, and particularly as a coating for sand used in such applications. In one such application, a solid novolac resin is used to coat a suitable foundry sand for use in manufacturing molds and cores as follows: the solid novolac resin, generally in flake form, is added to preheated sand (200° F. to 400° F.) (93° C. to 204° C.) in a suitable production coater, the temperature being above the melt point of the novolac (typically 170° F. to 200° F.) (77° C. to 93° C.). The heat causes the novolac resin to melt and the mixing action uniformly coats the surface of the sand. Subsequent to achieving uniform coating, an aqueous solution of hexa (hexamine) is added. The added water cools the coated sand through evaporation. This cooling rapidly drops the temperature of the coated sand and stops (prevents) the curing of novolac resin by the hexamine. The so-coated sand is discharged and further cooled and screened. This free-flowing, coated sand now has a thermosetting coating and can be molded into cores and molds.
In an alternative approach, instead of directly using a solid novolac which is heated to produce a melt, a liquid novolac also can be used to coat the sand particles. Such liquid novolac resins comprise a solution of the solid novolac resin in a suitable solvent, such as methanol or ethanol. The liquid novolac is prepared by adding a solvent to a melt of the solid resin. During the coating of the sand the added solvent is evaporated using the “warm air” process. The liquid novolac is added to sand, which may be at an ambient temperature or pre-warmed, allowed to mix and coat the sand for about one to two minutes. At this time, a powdered hexamine and wax, sometimes added as premixed together, are added and mixing is continued. Following the mixing, warm or hot air is blown onto the mixing, coated sand to evaporate any remaining solvent. The sand is discharged, cooled and screened to remove large lumps.
The aggregate most often used with the novolac foundry resin binder for making cores and molds is natural silica but other aggregates that can be similarly coated and used include zirconia, chromite, olivine, ceramics and fused silica (hereinafter referred to as foundry mold aggregates).
Novolac resins also find similar use in the coating of proppants used in connection with hydrocarbon recovery efforts. Proppants are used to hold open fractures created during hydraulic fracturing operations that are used to enhance the recovery of hydrocarbon deposits from subterranean formations. Proppants generally have a larger grain (particle) size than the aggregates that are used for manufacturing foundry shells and cores. Nonetheless, the coating process is very similar with the exception that in some applications the novolac-hexamine coating may be allowed to achieve full cure in the mixer prior to discharge of the coated particles. As recognized by those skilled in proppant technology, proppants can include silica, ceramics, bauxite, and lighter density materials such as walnut, and porous ceramics.
The novolac resins used in such coating operations generally contain a relatively large amount (for example, about 3 to 7% by weight) of free phenol, in part because of the need to use an excess of phenol as a starting material during the synthesis of the novolac resin. Free phenol generally will be in an excess of from about 10 to 20% following the completion of the reaction between phenol and formaldehyde, depending primarily on the mole ratio used to produce the novolac resin.
This excess phenol creates a potential environmental concern as the phenol is volatilized during both the coating operation and during the eventual cure of the coated particles when used to make the foundry shells and cores, and/or coated proppants.
Even so, the residual phenol in the solid novolac plays an important role, acting similarly to a plasticizer and solvent for the novolac resin allowing the resin to flow more freely as the novolac is heated to its molten state. Removing the free phenol completely (or even to lower levels) causes an opposite effect, reducing the ability of the novolac resin to flow as it is heated to a melt. For example, a resin exhibiting a melt viscosity of 3,000 cps at a 4% level of free phenol, may exhibit a melt viscosity above about 5,000 cps if the free phenol is reduced to about 1.5% or lower and will typically exhibit a melt viscosity of 6,000 to 7,000 cps or higher if the free phenol is reduced to about 1% or lower.
The quantity of free phenol may be reduced further and to some extent quite easily (for example, to about 1.5% and lower) by removing the residual phenol by any of several established techniques, including heating, preferably with a vacuum assist, azeotropic distillation and thin film evaporation. However, reducing the level of free phenol beyond the earlier mentioned limits is not done without the noted consequence. In particular, as the level of free phenol is reduced further the processability of the resulting novolac resin (usually provided in flake form) is compromised, especially the flow characteristics of the resin melt as relates to the coating of foundry aggregates and the subsequent cured binder strength and the ability to maintain crosslink density for coated proppants.
Thus, when the lowered phenol-content novolac resin is used to coat an aggregate, the higher melt viscosity of the resin prevents a more uniform coating. Resin melt viscosity will decrease, or the melt becomes thinner with increasing temperature and the sand temperature to be coated can be raised. And, while a higher coating temperature might be attempted, to counteract the higher viscosity characteristic of the melt at a particular temperature, this approach becomes counter-productive. The higher temperatures employed leads to premature curing of the resin in the presence of the added hardener and accordingly a loss of tensile strength in the eventual product, such as a sand core or sand mold, made from the novolac resin.
One prior art approach attempting to solve this problem has been to synthesize what is called a “greener” resin. A greener resin is a lower molecular weight resin made using a higher excess of phenol at the time the resin is synthesized. Or in other words, the mole ratio between the phenol and formaldehyde is increased. If the original resin mole ratio was 1:0.75 (P:F), then a greener mole ratio would be less than this value, say for example 1:0.70 (P:F). Because of its lower molecular weight, the “greener” resin exhibits a lower viscosity at a given level of free phenol. For example, one can synthesize a novolac resin that exhibits a viscosity of about 1,500 cps at a 4% level of free phenol. When distilled down to a free phenol content of about 1.5%, the novolac resin exhibits a viscosity in the range of 3,000 cps to about 4,000. Unfortunately, because of the relatively high cost of phenol relative to formaldehyde, using higher levels of excess phenol to synthesize the novolac resin is generally disfavored. More importantly, this “greener” approach also reduces the tensile strength and integrity of the cores and molds made from the resin-coated aggregates, by creating a lower cross-link density in the resin due to the lower molecular weight of the resin.
There is also a benefit in reducing the residual level of free phenol in novolac resins used to coat frac sand (and other aggregates) for the manufacture of resin coated proppants. In some hydrocarbon recovery operations, any free phenol in the resin that leaches out of the coated proppant during use may interfere with the frac fluid viscosity as the proppant is being pumped into the fracture zone of the subterranean formation. Just as in the case of a foundry resin binder, in order to have a novolac resin that still retains an adequate flowability at 0.1 to 0.2% free phenol during the procedure for coating the proppant, the mole ratio used to make the resin must be much lower. This lower mole ratio, again called a “greener” mole ratio, results in a much lower melt viscosity at 0.1 to 0.2% free phenol. The unfortunate drawback to this approach is that the lower mole ratio novolac resin produces a less densely cured network in the coating when the novolac is finally cured on the proppant. This less dense cross-linked network results in a lower strength in the coated proppant with a lowered ability to resist crushing from the geologic formation, in which the closure stress may be up to 8,000 psi and higher.
Thus, there is a continuing need in the novolac resin art for a method of reducing the level of free phenol in novolac resins without severely compromising the properties of the resin and especially the flow property of the molten resin as it is heated, especially as it applies to the coating of foundry aggregates and proppants and their subsequent performance.